PROJECT SUMMARY TDP-43 is a DNA/RNA binding protein that serves as the global regulator of cellular RNA metabolism. Although TDP-43 has been extensively studied in neurons with regard to neurodegenerative diseases, its role in oligodendrocytes has never been addressed. The loss of TDP-43 function, as indicated by nuclear clearance of TDP-43 and proteinopathy, in oligodendrocytes has been observed in many neurodegenerative diseases as indicated by nuclear clearance of TDP-43 and proteinopathy. Mature oligodendrocytes in the brain play a diverse set of functions, including generating myelin sheaths along neuronal axons to allow saltatory conduction of action potentials, providing metabolic support to neurons and modulating extracellular K+ homeostasis. It remains to be determined if TDP-43 dysfunction in oligodendrocytes disrupts oligodendrocyte function and further contributes to the pathophysiology of neurodegenerative diseases. Preliminary studies using transgenic mice with a genetic deletion of TDP-43 specifically in mature oligodendrocytes revealed that the loss of TDP-43 confers structural abnormalities in oligodendrocytes that may be visualized histologically and via in vivo two-photon imaging. These structural changes involved the presence of aberrant cytoplasmic processes that form loops and tangles and inappropriately associate with neuronal somata and blood vessels. Given that oligodendrocytes normally show an exquisite selectivity of myelination in vivo, where only neuronal axons associate with oligodendrocyte processes, it is intriguing how the loss of TDP-43 induces such dramatic deficits in oligodendrocytes to recognize environmental and cellular cues to regulate their target-selectivity. Furthermore, the mice with oligodendrocyte-specific TDP-43 deletion exhibited spontaneous seizures and accelerated death accompanied by deficits in motor behaviors. The overarching hypothesis of this proposal is that the loss of TDP-43 in oligodendrocytes disrupts morphological structures, target-selectivity and function that ultimately culminate in neuronal hyperexcitability, neurodegeneration and neuroinflammation. I will test this hypothesis by employing a specific genetic deletion of TDP-43 in oligodendrocytes, conducting in vivo two-photon imaging, performing extensive histological analyses, and using electrophysiology and video-EEG. In Aim 1, I will determine if the selective loss of TDP-43 affects the dynamics of oligodendrocytes and oligodendrocyte lineage cells. In Aim 2, I will further characterize and define the aberrant interaction of abnormal oligodendrocyte processes with neuronal cell bodies and blood vessels. Lastly, in Aim 3, I will determine if the dysfunction of oligodendrocytes due to the loss of TDP-43 results in the alteration of neuronal excitability, neurodegeneration and neuroinflammation. These studies will extend our knowledge about the cell-type specific role of TDP-43 and how TDP-43 dysfunction in oligodendrocytes contributes to a wide array of neurodegenerative diseases that exhibit TDP-43 pathology in neurons and oligodendrocytes.